This application is a national stage of International Application No. PCT/DE2006/002189, filed Dec. 8, 2006, which claims priority under 35 U.S.C. §119 to German Patent Application No. 10 2005 061 752.2, filed Dec. 21, 2005, the entire disclosure of which is herein expressly incorporated by reference.
The use of piezo-electric elements is generally known. Such piezo elements are used, for example, to detect deformations in components by mounting them on components so that they move along with the deformation, causing a charge transfer in the piezo element. On the other hand, piezo elements are also used to influence (deform) a component in a targeted manner; that is, the piezo element is supplied with voltage and the resulting deformation is utilized. Piezo elements are used mainly when special complex deformation processes are required on the component and the surface of the components should be as undamaged and smooth as possible in all deformation conditions. Examples of applications exist, for example, in aeronautical engineering in the case of aerodynamic profiles, and also for large concave mirrors, for example, in telescopes, and more.
In the field of aeronautical engineering, piezo elements are used to examine the flow around aerodynamic profiles. German Patent Document DE 103 04 530 A1 describes an arrangement in which piezo actuators are inserted at least in parts in an aerodynamic profile. When the piezo actuators are acted upon electrically, they change length, essentially in the direction of the planes of the cover skins of the profile. The profile has a forward profile region and a rearward profile region situated in the downward current, and is bounded by pressure-side and suction-side cover skins which converge in a trailing edge of the profile. In addition to circuit connections, the piezo-electric actuators contain piezo elements with a so-called longitudinal effect (d33 effect), in which the change of length of the piezo-electric material takes place in the direction of the electric field, permitting an effective introduction of forces into the aerodynamic profile. In the piezo-electric actuators used in German Patent Document DE 103 04 530 A1, which utilize the d33 effect, the change of length of the piezo-electric material is in the direction of the electric field, and is greater than the piezo effect (d31 effect), in which the change of length takes place perpendicular to the electric field.
The d33 actuators used in German Patent Document DE 103 04 530 A1 are produced by cutting slices having a thickness d out of a stack-shaped piezo element, in the longitudinal direction, to form flat disks as shown in FIG. 1a. The latter are then placed onto or into a curved structure, specifically the aerodynamic profile. The actuators have a narrow thickness and are essentially plate-shaped or are flat rectangular parallelepipeds, so they do not influence the aerodynamic conditions or influence them only little. However, since the piezo-electric actuator is to be mounted on curved or shaped profiles (or is to be placed in the latter) and is to generate aerodynamic resistance at the curved aerodynamic profile, the piezo elements contained in the piezo actuators frequently have to be bent or curved. As a result, they break easily during the adaptation or mounting on such curved structures, especially because the d33 piezo materials are relatively brittle. In addition, the layers of the piezo element may possibly be displaced with respect to one another or deformed, which in turn may affect the precision and operating capacity of the piezo material.
Conventional d31 piezo elements are therefore frequently used. In such elements, the change of length takes place perpendicular to the electric field, and they therefore have a thinner design and are more flexible with respect to deformation. However, the piezo effect (the achievable deformation) is less, so that the performance of the d31 piezo elements is often not satisfactory for influencing the components in a targeted manner.
Based on the above, one object of the invention is to provide a piezo element (and an actuator having such a piezo element for influencing a mechanical component), which is a high-performance actuator and is adapted to the shape of the component, the shape of the occurring load, and/or the load to be applied.
This and other objects and advantages are achieved by the piezo element according to the invention, in which a surface of the stack-type piezo element is shaped to correspond to a surface of the initial stack-type piezo element in a rectangular parallelepiped shape, which surface extends perpendicular to the layer planes of the stack (parallel to the stacking direction). As a result, the stack-type piezo element may have a three-dimensional shaping and can be adapted to the design of an aerodynamic profile. Because the piezo effect occurs perpendicular to the layer planes, (that is, in the stacking direction), the shaping of the piezo element does not affect its performance.
A shaped surface means that the surface is, for example, not planar; that is, one lateral surface of the rectangular parallelepiped of the initial stack of a stack-type piezo element is replaced, for example, by a curved, wavy or otherwise designed surface. As an alternative, a plane surface may also form the shaped surface which, however, it is disposed at a nonzero angle with respect to the stacking direction, resulting, for example, in a piezo element which, as a whole, has the shape of a prism. Instead of two mutually opposite parallel surfaces of the stack, in this case, the two opposite surfaces are disposed relative to one another at an angle that is neither 0° nor 90°.
In each case, the layers of electrically conductive material that form the stack-type piezo element are not all shaped the same. That is, shaping of a surface means that, in a “virtual” disassembly of the stack-type piezo element into the individual conductive layers, the individual plates of the stack would have different plate shapes. There is no special limitation for the design of the surfaces. They can, on the contrary, be adapted to the corresponding application of the piezo element as required.
A shaped surface is therefore any two-dimensionally (2D) or three-dimensionally (3D) machined stack element. Two-dimensional machining refers to machining in one plane of the stack-type piezo element, which leads to a stack of a varying thickness, while three-dimensional machining indicates a machining of the stack-type piezo element in several planes, from which an almost arbitrarily contoured stack-type piezo element is created which has freely designable ascending and descending shapes. In three-dimensional machining, the contour of the piezo element is a function of all three directions in space, while, in the case of the two-dimensional machining, the contour of the piezo element does not vary in one of the three dimensions in space.
Shaping of the shaped surface is performed after the construction of the piezo element. A machining process, (such as, for example, a sawing, grinding, drilling, turning, broaching, lapping or milling process, or a combination of theses processes) is used.
Thus, for a d33 piezo actuator, conventionally, first a piezo element may be constructed as a stack; that is, in a form that is not adapted to the shape (for example, as a rectangular parallelepiped with two approximately square lateral surfaces which simultaneously are layer plane surfaces). Subsequently, before it is mounted on a component or used as an actuator, at least one surface of the stack is adapted to the shape of the component, to the expected loading of the piezo element, to the load that is to be placed by the piezo element, or to a combination of these demands. To this end, at least one surface of the rectangular parallelepepid shaped stack, which surface is parallel to the stacking direction, is machined, for example, mechanically.
For aerodynamic applications, it may be preferred that the shape-adapted surface of the piezo-electric actuator, specifically the surface facing the exterior side of an aerodynamic profile, be curved so that it corresponds to the profile contour. As a result, the aerodynamic shape of the profile can essentially remain unaffected, while at the same time a piezo element is provided for influencing of the aerodynamic component. In the thickness direction, the piezo element may, for example, have a constant dimension, which means that the surface situated opposite the shaped surface is also correspondingly shaped. Thus, for example, the piezo element may have a concavely curved and a convexly curved exterior surface. As an alternative, the piezo element may have a variable thickness in that, for example, either no shaping or a different shaping is performed at the other surfaces.
By varying its thickness, the piezo element can, for example, be adapted to loads which occur at the component and are introduced into the piezo element. Also in the case of a three-dimensional profile, a three-dimensional influencing of the component can be achieved by means of a variable thickness of the piezo element.
The piezo element preferably is a d33 stack-type piezo element, in which the piezo effect occurs in the direction perpendicular to the stack layers (that is, in the stacking direction). Because of the shaping of the actuators or of the stack-type piezo elements, the actuators do not have to be further bent or deformed during the installation. Thus, the risk of breakage during the installation (for example, by means of gluing, clamping or screwing), as a result of the bending as well as a deformation of the layers or of the layers with respect to one another, is avoided. Therefore, the capacity of the piezo actuator is maintained and the wear of the piezo actuators is reduced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.